(a) Field of the Invention
This disclosure relates to a transparent conductive layer and a method of manufacturing the same.
(b) Description of the Related Art
A photoelectric device, such as a solar cell, converts light energy into electric energy. Types of photoelectric devices can be differentiated by the metals used therein for the active layers. Thus a solar cell converts solar energy into electric energy, and generates electricity using at least two kinds of semiconductors, a P-type semiconductor, and an N-type semiconductor.
Classes of solar cells include crystalline silicon solar cells, which are commercially available, thin film solar cells, which are based on low cost substrates, and composite solar cells, which can be a crystalline silicon-type solar cell or a thin film-type solar cell.
Crystalline silicon solar cells, in which slices of silicon ingots are used as substrates, are classified as monocrystalline solar cells or polycrystalline solar cells, depending upon the silicon processing method. A monocrystalline silicon solar cell has a PN junction structure including an N-type semiconductor, which includes a pentavalent element such as phosphorous, arsenic, or antimony doped into the silicon, and a P-type semiconductor, which includes a trivalent element, such as boron or gallium doped into the silicon. The resulting structure is roughly the same as that of a diode.
A thin film solar cell can be formed by disposing a film on a substrate, which includes glass or plastic. In commercially available thin film solar cells, the diffusion distance of carriers is very short due to the characteristics of the thin film, as compared to crystalline silicon solar cells. Also, if the thin film solar cell is fabricated only with a PN junction structure, the collection efficiency of light generated electron-hole pairs is low. Therefore, a thin film solar cell can include a PIN structure wherein an intrinsic semiconductor-based light-absorbing layer with a high light absorption is interposed between a P-type semiconductor and an N-type semiconductor. Commercially available thin film solar cells include a structure where a front transparent conductive layer, a PIN layer, and a rear reflective electrode layer are sequentially disposed on a substrate. In this structure, the light-absorbing layer is depleted due to the overlying P and the underlying N layers, which include a high doping concentration, so that an electric field is generated therein. As a result, when light, such as sunlight, generates a carrier in the light-absorbing layer, an electron is collected at the N layer and a hole is collected at the P layer by way of drift of an internal electric field, thereby generating an electric current.
In a solar cell, the light-absorbing layer includes a multi-component compound such a Si, GaAs, CdTe, or CuInSe2. Because silicon is an indirect transition material, the light absorption coefficient of silicon is very low compared to that of other compounds, such as CdTe or CuInSe2. For this reason, where the light-absorbing layer is disposed as a thin film including a thickness of several microns or less, it does not absorb all of the incident light, and therefore current density loss occurs due to transmitted light.
A textured transparent conductive layer may be used to enhance the efficiency of the solar cell. A textured transparent conductive layer can increase a distance light must travel because of light scattering, thus improving light absorption and significantly enhancing an efficiency of the solar cell. However, a current textured transparent conductive layer preferentially scatters short wavelength light, thus scatters long wavelength light less.
Accordingly, an improved light scattering or trapping technique including a front transparent conductive layer and a rear reflective electrode, which can scatter loner wavelength light, would be desirable to improve the efficiency of a solar cell.